An image forming apparatus employing an electrostatic electrophotography printing method generally carries out steps of charging, exposing, developing, transferring, cleaning, charge removing, and fixing, so as to form an image. In a charging step, a surface of a photoreceptor which is driven to rotate is uniformly charged by a charging device. In an exposing step, the charged surface of the photoreceptor is irradiated with a laser beam emitted from an exposure device so that an electrostatic latent image is formed. Then, in a developing step, the electrostatic latent image formed on the photoreceptor is developed with a developer by a developing device so that a toner image is formed on the surface of the photoreceptor. After that, in a transferring step, the toner image formed on the photoreceptor is transferred to a recording material by a transfer device. Then, in a fixing step, the recording material is heated and pressured by a fixing device so that the toner image is fixed on the recording material. Further, in a cleaning step, a residual toner that has net been transferred in the transferring and remains on the surface of the photoreceptor is removed by a cleaning device so as to be collected into a predetermined collection section. Then, in a charge removing step, a residual charge is removed, by a charge removing device, from the surface of the photoreceptor that has been subjected to the cleaning step. The image forming apparatus becomes then ready for formation of a next image.
The developer used to develop the electrostatic latent image formed on the photoreceptor in the developing step is generally a single-component developer made of only a toner, or a two-component developer made of a toner and a carrier. The single-component developer contains no carrier. For this reason, with the single-component developer, it is unnecessary for the developing device to employ a stirring mechanism for mixing a toner and a carrier with each other uniformly, or the like. Accordingly, the single-component developer has an advantage of a simple arrangement of the developing device. However, the single-component developer also has disadvantages, such as an inconstant charge amount of the toner. On the other hand, with the two-component developer, it is necessary for the developing device to employ the stirring mechanism for mixing a toner and a carrier with each other uniformly, or the like. For this reason, the two-component developer has a disadvantage of a complex arrangement of the developing device. However, the two-component developer is excellent in stability of a charge amount of the toner. Accordingly, the two-component developer is now suitably used in devices such as a high-speed image forming apparatus and a color image forming apparatus.
In a case where an image forming apparatus uses such a two-component developer, it is necessary to, inside a small space, (i) charge the developer rapidly and sufficiently and also (ii) carry the developer rapidly since there has been a demand for a high-speed, downsized image forming apparatus. In view of this, Patent Literature 1 discloses an image forming apparatus which includes a circulation-type developing device. As disclosed in Patent Literature 1, the circulation-type developing device includes (i) a developer carrying path through which a developer in a developing tank (developing container; developer container) is carried circularly and (ii) a developer carrying member for carrying and stirring the developer in the developer carrying path. According to Patent Literature 1, the developer carrying member is a screw including (i) a blade having a feed screw shape and (ii) a mesh screen member.